Load reduction of a visitor location register

ABSTRACT

A problem arises when the data of the visitor location register (VLR) are lost for some reason. In this case, only the visitor location register (VLRn) in whose area the mobile station (MS) is located is known. The mobile station (MS) must then be paged in all location areas (LAn) of the visitor location register (VLR). Such paging of mobile stations causes a significant overload. The overload is reduced, according to the invention, by dividing the area covered by a physical visitor location area (VLR) into areas of a plurality of logical visitor location registers (LVLRn). In connection with location updatings, not only the physical visitor location register (VLR) is stored by also information on the logical visitor location register (LVLRn) in whose area the mobile station (MS) is located; in a problematic situation where the switching centre (MSC) is not aware of the location area of the mobile station(MS), the mobile station (MS) is paged, at least at first, only in the area of the logical visitor location register (LVLRn).

The invention relates to mobile communication systems and particularlyto reducing the load of their visitor location registers.

FIG. 1 illustrates the roaming of a mobile station MS in a mobilecommunication network or in a network subsystem NSS. In this example,the NSS comprises six base transceiver stations BTS1 to BTS6, which canbe controlled by one or more base station controllers BSC. Each basestation BTS serves one cell. When an MS terminated call is switchedthrough the network NSS, the NSS must know the area in which the MS isto be paged. The area in which the MS is paged is called a location areaLA.

The size of the location area LA is a compromise. If the location areais very large, the MS must be paged in the area of several base stationsBTS. This means that mobile stations are also paged in base stationareas where they are not located. This consumes the capacity of radiochannels. If the location area is small, location updatings must beperformed frequently. FIG. 1 shows, by way of illustration, threelocation areas LA1-LA3, which cover 1, 2 and 3 base stations,respectively. The base stations BTS continuously broadcast informationon themselves and their environment, such as a base station identifierBSI and a location area identifier LAI. On the basis of the LAI, an MSregistered in a BTS knows in which LA it is located at each moment. Ifthe MS, when changing the BTS, observes that the LAI has changed, itsends a location update request or message to the network NSS. The LA ofthe MS is updated in the visitor location register VLR in whose area theMS is located at that moment. The VLR typically stores e.g. thesubscriber's IMSI (International Mobile Subscriber Identity) number,MSISDN number, subscriber service data and LAI.

Information on the VLR in whose area the MS is located is sent to thehome location register of the MS. The HLR stores permanent subscriberdata, which are not dependent on the subscriber's location at a givenmoment: e.g. the subscriber's MSISDN and IMSI, subscriber service data,and routing data to the VLR in whose location area the MS is located.

FIG. 2 illustrates the setting up of an MS terminated call in a GSM-typemobile communication system. If the MS of FIG. 1 is not located withinthe area of its own home location register, the parts of FIG. 2correspond to those of FIG. 1; however, the HLR and VLR are associatedwith different mobile services switching centres MSC. In step 1, a callarrives at a gateway mobile switching centre GMSC. In step 2, the GMSCdefines the subscriber's home location register HLR on the basis of thecalled subscriber's directory number MSISDN, and sends it a request forrouting data. Information on the VLR in whose area the subscriber islocated is updated in the subscriber's HLR in connection with locationupdating. On the basis of this information, the HLR sends the VLR a`Provide Roaming Number` request in step 3. The IMSI of the MSsubscriber is also sent to the VLR in this request. In step 4, the VLRstores the data it has received and assigns a mobile station roamingnumber MSRN. In step 5, the VLR sends the MSRN it has assigned to theHLR, which in step 6 sends it to the switching centre GMSC thatrequested the routing data. The roaming number domain is defined in sucha way that a call is always directed to the MSC whose VLR has assignedthe roaming number in question. On the basis of the MSRN, the GMSC isthus able to route the call forward by sending an initial addressmessage in step 7 to the MSC indicated by the MSRN.

In this example, the MSC finds out on the basis of the MSRN that thecall will terminate in its own area. In step 8, the MSC requests thedata of the called subscriber from its own VLR call set-up. In a normalcase, the VLR returns the requested data in step 9 for setting up thecall. This is indicated by arrows 10 and 11.

A problem arises when the data of the VLR are lost for some reason. Thismay happen, for instance, when the VLR is reinitialized, possibly duringsoftware updating or equipment maintenance. In this case, only the VLRin whose area the MS is located is known (on the basis of the HLR of theMS). The MS must then be paged in all location areas LA where there aremobile stations whose data are stored in the VLR in question. If the VLRserves a large number of subscribers, such paging of mobile stationscauses a significant overload mainly in three areas: 1) between the MSCand the base station system BSS; 2) in the signalling process of theBSS; and 3) on radio channels. The BSS comprises a base stationcontroller BSC and base stations BTS controlled by the BSC. As a resultof the overload, some of the MS terminated calls fail.

The object of the invention is thus to provide a method and an apparatusimplementing the method in such a manner that the above-mentionedoverload problems are solved. The objects of the invention are achievedwith methods and systems characterized by what is disclosed in theindependent claims. Preferred embodiments are disclosed in the dependentclaims.

The invention is based on dividing the area of a physical VLR into areasof a plurality of logical VLRs. When a mobile station performs locationupdating, the location update request sent by the VLR to the HLR alsocontains information on the logical VLR in whose area the mobile stationis located. This information is transparent to the HLR, i.e. the HLRstores it but does not react to it in any way.

In the case of an incoming call, signalling takes place as before. In aproblematic situation where the VLR is not aware of the location area ofthe MS, the physical VLR receives the address of the logical VLR of theMS from the HLR with normal signalling. This information is included ina `Provide Roaming Number` message. The physical VLR thus limits thepaging to the area of the logical VLR. In the most extreme case, thearea of the logical VLR may even be only one location area.

It is an advantage of the method and system of the invention that thenetwork is considerably less loaded in problematic situations than inprior art systems. Another advantage of the arrangement of the inventionis that the modifications to be made in an existing system arerelatively small, for instance modifications in the MSC software. Themodifications are compatible with the existing systems in every respect.A network can thus employ--simultaneously and in a flexible manner--bothswitching centres modified according to the invention and switchingcentres not modified according to the invention.

In the following, the invention will be described in greater detail bymeans of preferred embodiments with reference to the accompanyingdrawings, in which

FIG. 1 shows the parts of a mobile telephone network that are essentialto the invention;

FIG. 2 shows the steps of a signalling process of a mobile telephonenetwork that are essential to the invention; and

FIG. 3 shows location areas of a mobile telephone network, divided intoareas of a plurality of logical visitor location registers.

With reference to FIG. 3, the overload of a mobile telephone network islimited, according to the invention, by dividing the area of a physicalvisitor location register VLR into areas of a plurality of logicalvisitor location registers LVLRn. For reasons of clarity, the area ofthe physical VLR is divided in FIG. 3 into areas of only two logicalvisitor location registers LVLR1 and LVLR2. In practical applications, asuitable number n of LVLRs in the area of one physical VLR could be, forexample, 5 to 10. The system of FIG. 3 would require the followingmodifications in the prior art solution of FIGS. 1 and 2:

1. Defining and naming LVLRs of suitable size. In the naming, it issufficient to use, for example, 1 or 2 digits after the number of theVLR.

2. The message sent by the MSC to the HLR in connection with thelocation updating of an MS also contains information on the LVLR inwhose area the MS is located. This information is transparent to theHLR, i.e. the HLR processes it as an ordinary number of a physical VLRwhich is only 1 or 2 digits longer than it would be without theidentifier n of the LVLR.

3. In the case of an MS terminated call, the changes will not becomevisible until the data of the VLR have been lost for one reason oranother, i.e. the VLR does not know the location area of the MS. In thiscase, the MSC (and its VLR) receives the identifier of the LVLR of theMS from the HLR in connection with normal signalling (in a `ProvideRoaming Number` message). On the basis of this information, the MSC maylimit the paging of the MS to only the area of the LVLR.

According to the prior art, the following information, for example, isstored in a HLR:

    ______________________________________                                        an MS whose IMSI number                                                                           is in the area of a VLR                                   is                  whose GT address is                                       IMSI                NCC NDC VLRn                                              ______________________________________                                    

where:

GT=Global Title

NCC=National Country Code

NDC=National Destination Code

VLRn=Visitor Location Register number.

According to the invention, the following information is correspondinglystored in the HLR:

    ______________________________________                                        an MS whose IMSI number                                                                           is in the area of a VLR                                   is                  whose GT address is                                       IMSI                NCC NDC VLRn nn                                           ______________________________________                                    

where

nn=the number of the LVLR (1 or 2 digits) within the physical VLRn.

The specification of a VLR as LVRLs is implemented according to theinvention for instance as software modifications in the MSC. Thesoftware modifications are as follows:

1. The MSC must keep a log of which location areas LAn belong to whichlogical visitor location registers LVLRn.

2. In connection with location updating of an MS, the MSC must also sendthe HLR of the MS information on the LVLRn in whose area the MS islocated.

3. In problematic situations where the data of the physical VLR havebeen lost, the MSC must, when paging the MS, at least at first limit thepaging to only the LVLRn area where the location of the MS has last beenupdated.

Dividing a visitor location register VLR into (or specifying a VLR as)logical visitor location registers LVLRn reduces the congestion peaksoccurring in problematic situations. If the number n of LVLRs within thearea of a physical VLR is five, for example, the MS must be paged in anarea that is five times smaller, and the load caused by the paging isreduced accordingly. There is, nevertheless, a side effect: the numberof location updatings grows by the same coefficient. However, theadvantages resulting from the reduction of congestion peaks outnumberthe drawbacks caused by the additional traffic caused the locationupdatings.

It will be obvious to one skilled in the art that the examples describedabove are in every respect intended to illustrate the invention, not torestrict it. The invention has been described, by way of example, withreference to a GSM system, but it may also be applied to other kinds ofmobile communication systems. The invention and its embodiments are thusnot limited to the examples described above, but they may be modifiedwithin the scope of the appended claims.

What is claimed is:
 1. A method for updating location of a mobilestation (MS) in a digital mobile telephone network (NSS), wherein onehome location register (HLR) stores information on the physical visitorlocation register (VLR) in the area of which the mobile station (MS) islocated, characterized in thatthe area covered by a physical visitorlocation register (VLR) is divided into areas of a plurality of logicalvisitor location registers (LVLRn); and the home location register (HLR)stores, in addition to the physical visitor location register (VLR),information on the logical visitor location register (LVLRn) in whosearea the mobile station (MS) is located.
 2. A method according to claim1 characterized in that said physical visitor location register (VLR) isdivided hierarchically, whereby each logical visitor location register(LVLR) is subject to a single physical visitor location register (VLR).3. A method according to claim 2, characterized in that said areas ofsaid logical visitor location registers (LVLRn) are formed on the basisof geographical location, each area preferable comprising a smallinteger number of location areas.
 4. A method according to claim 1,characterized in that said areas of said logical visitor locationregisters (LVLRn) are formed on the basis of geographical location, eacharea preferably comprising a small integer number of location areas. 5.A method for paging a mobile station (MS) in the area of a mobileservices switching centre (MSC) in a digital mobile telephone networkwherein, at any time, the location area (LA) of a mobile station (MS) ismaintained in one Visitor Location Register (VLR) and the information onthe VLR is maintained in one Home Location Register (HLR), wheninformation on the location area (LA) of the mobile station (MS) hasbeen lost by the switching centre (MSC), characterized in thatthe areacovered by the physical visitor location register (VLR) of the switchingcentre (MSC) is divided into areas of a plurality of logical visitorlocation registers (LVLRn); and the mobile station (MS) is paged, atleast at first, only in the area of the logical visitor locationregister (LVLRn) last indicated by the mobile station.
 6. A methodaccording to claim 5, characterized in that said physical visitorlocation register (VLR) is divided hierarchically, whereby each logicalvisitor location register (LVLR) is subject to a single physical visitorlocation register (VLR).
 7. A method according to claim 6, characterizedin that said areas of said logical visitor location registers (LVLRn)are formed on the basis of geographical location, each area preferablecomprising a small integer number of location areas.
 8. A methodaccording to claim 5, characterized in that said areas of said logicalvisitor location registers (LVLRn) are formed on the basis ofgeographical location, each area preferable comprising a small integernumber of location areas.
 9. A mobile services switching centre (MSC) ina digital mobile telephone network wherein, at any time, the locationinformation of a mobile station (MS) is maintained in one VisitorLocation Register (VLR) and the information on the VLR is maintained inone Home Location Register (HLR), said mobile services switching centre(MSC) comprising a physical visitor location register (VLR) and beingadapted to transmit information to a home location register (HLR) of themobile telephone network on the physical visitor location registers(VLR) in the area of which each mobile station (MS) is located,characterized in thatthe area covered by the physical visitor locationregister (VLR) of the switching centre (MSC) is divided into areas of aplurality of logical visitor location registers (LVLRn); and theswitching centre (MSC) is also adapted to transmit information to thehome location register (HLR) on the logical visitor location registers(LVLRn) in the area of which each mobile station (MS) is located.
 10. Alocation update message in a digital mobile telephone network (NSS)wherein, at any time, the location information of a mobile station (MS)is maintained in one Visitor Location Register (VLR) and the informationon the VLR is maintained in one Home Location Register (HLR), saidlocation update message containing information on the visitor locationregister (VLR) in whose area the mobile station (MS) is located,characterized in that the location update message further containsinformation on the logical visitor location register (LVLRn) in the areaof which the mobile station (MS) is located.